I'm not sure what you mean by "nick".... is it crack or gouge/indentation? I'm a little confused because you also mention tearing...
If it is a crack, I would use a high grade "super glue" to provide ridgity to the crack. Then use an epoxy to fill the void. You have to be patient and careful with the epoxy. Use the 24 hour type rather than the fast setting.
Mask off the area with tape and fill the void generously. Wait for it to partially set. When it becomes semi solid like a stiff gel but still sticky to the touch use a new razorblade placed parallel to the edge of the surface and cut off the excess. The technique should be similar to how you would scrape old adhesive/labels across a flat sheet of glass. Be very careful not to cut into the plastic shell with the blade.
If the void you are filling is part of a bevelled corner, use wax paper to gently mold the epoxy (after cutting off excess) with your hand. With patience and care you should be able to restore all of it. If you are picky you can then use very very fine grade sand paper to polish the epoxy so that it is consistent with the rest of the surface.
The tricky part is colour matching. The pieces I've worked with fortunately were black or dark charcoal grey so I used a black permanent marker. If it is white..... you might try using enamel nail polish.
Derek